1. Field of the invention
The present invention relates to air-fuel ratio controller for an internal combustion engine, more specially, air-fuel ratio initial control for the internal combustion engine which controls air-fuel ratio by feedback based on the multiplication of a basic fuel injection duration determined by the load of the engine and engine speed, an air-fuel ratio feedback correction coefficient obtained by output signal of an oxygen sensor and a learning value which is variable so that the mean value of air-fuel ratio feedback correction coefficient is remained whithin the measure value preset range.
2. Prior Art
Generally, three-way catalyst has been used for simultaneous purification of carbon monoxide, hydrocarbon and nitrogen oxide in exhaust gas. To improve purification ratio of the catalyst, feedback control is applied to presume and control the air-fuel ratio to be in the vicinity of stoichiometric air-fuel ratio by detecting the concentration of residual oxygen in exhaust gas. To operate the feedback control, the fuel injection interval TAU is to be obtained by the multiplication of a basic fuel injection interval TP determined by the load of the internal combustion engine (intake pressure PM or intake air amount A/Ne per revolution) and engine speed, and air-fuel ratio feedback correction coefficient FAF, as shown in FIG. 6 which proportionaly integrate the fuel injection time interval according to the air-fuel ratio signal generated and processed by the signal from the oxygen sensor, which provides the opening position of the fuel injeciton valve during the time equivalent to that of TAU to control the air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio. The above stoiciometric air-fuel ratio feedback correction coefficient FAF delays to overtake the rapid change of the internal combustion engine operation, which causes the period when the air-fuel ratio is off the target air-fuel ratio. Changes in environment or lapse have caused variations of valve timing due to the variation of tapet clience, characteristics of pressure sensor, air-flow meter and fuel injection valve, which might fail in controling the fuel injection volume to the required volume for the engine to control the air-flow ratio in the vicinity of the stoichiometric air-flow ratio. To solve the problem, learning control for air-fuel ratio is adopted to remain the air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio. As shown in the following, the learning control adjusts the mean value of the air-fuel ratio feedback correction coefficient FAFAV to be preset value with learning value KG learned by the given condition EQU TAU=TP.times.KG.times.FAF(1+F(t))
where F(t) stands for the correction coefficient of increment of warming up or starting and is set up to 0.0 in feedback controlling of air-fuel ratio. The learning value KG is learned and updated every section according to the load of the internal combustion engine, for example, when intake air amount is 15-30 l/h, 30-45 l/h or 45 l/h-60 l/h, it is learned as KG1, KG2, and KG3, respectively.
These learning values KG (KG1, KG2, KG3) are learned with the following method whenever the correction coefficient FAF skips preset times in air-fuel ratio feedback controlling and cooling water temperature exceeds the preset value (for example, 80.degree.). At first whenever air-fuel ratio feedback correction coefficient FAF skips preset times, the arithmetrical mean FAFAV of the maximum/minimam value of FAF is to be obtained as follows: EQU FAFAV=(A+B)/2,(B+C)/2,(C+D)/2 . . . (2)
When the mean value FAFAV becomes out of the preset range (for example, a range of .+-.2% to the value of the stoichiometric air-fuel ratio), learning value KG isadjusted to be given value by learning. When a mean value FAFAV is above 1.02, the learning value KG is increased to a given value and the mean value FAFAV is below 0.98, the learning value KG is decreased to the given value.
The above-mentioned learning value KG applied to above equation (1) dependent on wheather the intake throttle valve is open or closed and intake air amount per revolution of internal combustion chamber, which provides TAU. As a result, when the mean valve FAFAV is above 1.02, the learning value is increased to control the air-fuel ratio to rich side, and the mean valve FAFAV is below 0.98, the learning value is decreased to control the air-fuel ratio to lean side, which results that the mean value FAFAV is learning controlled to approach the stoichiometric air-fuel ratio keeping its value as 1.
For example, the air-fuel ratio controller prevents air-fuel ratio feedback correction coefficient from changing and greatly improve transient characteristic of air-fuel ratio control even if the operating condition of the internal combustion engine changes rapidly because the most suitable learning value KG1, KG2 or KG3 is selected to be applied to the above equation (1). In case of secualr change in internal combustion engine characteristic, the mean value FAFAV of air-fuel ratio feedback correction coefficient FAF is invariable remaining in the vicinity of 1.0 and the change of fuel injection time TAU reflected by the secular change is absorbed by the learning value KG.
However, the above air-fuel ratio controller has caused following disadvantages.
Since the learning value KG is finite which is limited in designing control system as well as the feedback correction coefficient FAF, it is necessary for complete absorption of the secular change of the internal combustion engine as abovementioned to adjust the learning value KG to approximately the center value of the variable region in the initial state to be available for the change due to great increase or decrease of KG as much as possible.
Therefore, in the initial operation of the internal combustion engine, sensor output for determining the basic fuel injection time TP in shipment of the vehicle, for example, output from the air-flow meter is controlled to slightly adjust the value of the TP under the same operating condition and to adjust the learning value KG to the required value. That is, in the above equation (1), TP in the right side is varied to adjust KG to the value (generally 1.0) within the required range without varying the calculated fuel injection duration TAU.
The above adjustment is necessary for the initial operation of the internal combustion engine, however, such adjustrment requires a long duration compared with other conventional adjustments.
The learning value KG is determined by learning with the history of the variation of past air-fuel ratio feedback coefficient FAF and the variation of present air-fuel ratio feedback coefficient FAF, which makes it possible to provide reliable learning value KG contained no momentary disturbance. However, since the determination raquires a long-period-observation of the variety of the air-fuel ratio feedback correction coefficient FAF, the above adjustment vaires each variable in the above equation 1.
At first, for example, the output of the air-flow meter is adjusted to varry only the detected results of the sensor keeping factual operating condition of the internal combustion engine constant. Then, intake air-flow amount of internal combustion engine is judged to be varied, which causes the variation of the fundamaental fuel injection duration TP in accord with the variation. Since the acutal operating condition of internal combustion engine does not change, the variation of above TP causes the error of air-flow ratio of which the fuel injection duration TAU has changed. Therefore, the air-fuel ratio feedback correction coefficient FAF is calculated to adjust the detected error of the air-fuel ratio, at the same time, new learning value KG which is within a given range of air-fuel ratio feedback correction coefficient FAF is determined by updating the learning value KG from the observation results of past and present air-fuel ratio feedback correction coefficient FAF. As the determination of new learning value KG requires the period for the completion of learning to the new state, adjusting for the initial determination of the learning value KG has needed a long time. This adjustment not only deteriorates workability and efficiency of the stroke but also has a possibility to wrongly recognize the completing of the adjustment when the incompleted transient learning value KG momentary agrees with the given value, which has made the adjustment one of the most difficult among various ones.